1. Field of the Invention
There is a family of exercise machines that provides a manual workout task requiring a user to push or pull against a resistance provided by a stack of weight plates. The weight system is usually linked with a single cable to a gripping or user interface device to produce a constant resistance. A fracture of the tensioned cable along its length or at its end connectors causes a sudden acceleration of the grip or other interface device driven by the operator's push or pull. The sudden loss of resistance often results in an exerciser pulling a heavy bar into his or her face. Because falling weights, accelerating grips, and rapidly unloading muscles are all hazardous, manufacturers of exercise machines want to maintain the structural integrity of the cables. To accomplish this, manufacturers usually recommend “scheduled servicing” of their cables. This preventive maintenance (PM) strategy is frustrated by nylon sheathing that hides cable damage or failures. Further, the swedged or silver soldered connectors often fail covertly by internal fatigue fractures. A more effective PM strategy has been adopted by many manufacturers called “scheduled replacement”; they advocate annual cable replacement. However, the owners of the machines may not follow a recommended replacement schedule in order to minimize costs or because of mere forgetfulness and/or lack of organization.
2. Description of Related Art
Hanging weights are generally used to provide a constant resistance over the full pulling range on fitness equipment. These weights are usually suspended by a cable mechanism that transfers a selected gravity load to various portions of a human body through handles or foot pedals. This is illustrated by the stripped down fitness machine 20 shown in FIG. 1. In FIG. 1 a user interface device 22, i.e., a pull down bar, is connected to a resistance load 24 including an adjustable weight plate stack by a cable 26. Manufacturers typically use an aircraft cable (e.g., seven strands with 19 wires per strand) that is 3/16 inch in diameter and covered in a nylon sheath. This highly redundant wire rope is typically composed of 133 wires, and incorporates end connectors that are usually swedged and/or silver soldered.
Fitness equipment manufacturers generally advocate PM techniques for maintaining the structural integrity of the cables. Preventive maintenance is defined as actions performed in an attempt to retain an item in a specified condition by providing systematic inspection, detection and prevention of incipient failure by repairing or replacing it. This concept must be contrasted with corrective maintenance which restores an item to a specified condition after it has failed. Two principal PM techniques are scheduled servicing and scheduled replacement.
Scheduled servicing is comprised of scheduled maintenance strategies which reveal incipient failures of items for the purpose of preventing system failures. Various servicing protocols are involved:
a) Inspection—detection of self-revealing deteriorating conditions of items composing the system.
b) Non-destructive testing—a body of testing techniques and methods which will not compromise the item tested.
c) Cleaning
d) Lubrication
e) Calibration
f) Adjustment
g) Repair (if required)
Daily or weekly visual inspection of belts and/or cables on fitness machines for any signs of wear, fraying, de-lamination and/or stretching can be very effective for identifying damaged sheathing; on the other hand, it is an impoverished procedure for identifying the onset of fatigue failure. Fatigue of wire rope typically manifests itself by successive failures of the constituent wires which form wire fractures, often called “fish hooks,” on the cable surface. Wire rope is a highly redundant tension member that communicates its compromised structural integrity before it fully separates (Barnett, Ralph L., “Doctrine of Manifest Danger,” Triodyne Safety Brief, Vol. 8, No. 1, September 1992). Unfortunately, the nylon sheathing that covers most fitness machine cables hides most wire failures. Furthermore, wire rope failure may occur inside of the end fasteners where no visual feedback is available.
Scheduled replacement involves the replacement of parts at predetermined times before failure. When the onset of failure of an item cannot be determined, scheduled replacement is the only PM strategy available. Reliability analysis techniques are required to develop replacement schedules that will minimize the number of items which fail. On the other hand, inefficient replacement or maintenance schedules may be adopted through trial and error or anecdotal observations. Most manufacturers recommend a scheduled replacement of belts or cables every one or two years.
Keeping track of the age of fitness machine cables requires discipline. Throwing away one-year-old cables with no apparent damage also goes against the grain of frugality and resource conservation. “Changing out” cables that are inspected daily is work. Many sizes of new cables must be ordered and inventoried, and operations must be interrupted while new cables are installed. It is easier to ignore scheduled replacement and replace only broken cables or ones with compromised appearance.
There is a need for an improved cable safety system that provides that reduces or eliminates the risk of injury due to cable failure while limiting cost by not replacing structurally sound cables.